<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    
  </style>
</head>
<body>
  
</body>
<script>

  console.log( undefined == null );

  console.log( undefined == NaN );
  console.log( NaN == null );

  console.log( Number(undefined) );
  console.log( Number(null) );
  console.log( Number(NaN) );

  var a = undefined;
  var b = null;
  var c = NaN;
  console.log( ++a );
  console.log( ++b );
  console.log( ++c );

  console.log( String( [1,undefined,2,null,3,NaN,4] ) );

  var obj = {
    a: undefined,
    b: null,
    c: NaN
  }
  console.log( JSON.stringify(obj) );

  // =================
  
</script>
</html>